Continuous extrusion using dynamic shoe positioning

ABSTRACT

A continuous extrusion machine has a chassis ( 1 ) supporting a wheel ( 2 ) for rotation by a motor. An endless groove ( 7 ) extends around the periphery of the wheel ( 2 ). A shoe ( 3 ) is mounted in the chassis ( 1 ) and has an enveloping surface shaped to closely envelop an arc of the wheel ( 2 ) periphery so that the groove ( 7 ) co-operates with the shoe ( 3 ) to form a passage. An abutment is mounted on the shoe ( 3 ) to extend into the passage at a downstream end. Tooling is mounted in the shoe ( 3 ) including a die such that a material such as aluminium or copper bar fed into the groove ( 7 ) is extruded through the die as a consequence of the energy transfer via friction from the rotating wheel ( 2 ). A gap ( 12 ) exists between the enveloping surface and the wheel ( 2 ). The gap ( 12 ) is used to provide the orifice of a sonic gap ( 12 ) sensor whereby the size of the gap ( 12 ) can be accurately and directly measured. The gap ( 12 ) size sensed is used to control the position of the shoe ( 3 ) in two directions mutually perpendicular to the rotary axis of the wheel ( 2 ) by adjusting support structures which support th shoe ( 3 ). The size and shape of the gap ( 12 ) can thus be safely adjusted while the machine is extruding allowing the size and shape of the gap ( 12 ) to be adjusted for optimum performance.

[0001] The present invention is concerned with a continuous extrusionmachine, and a method of operation for continuously extrudingnon-ferrous metals such as aluminium and copper.

[0002] In general a continuous extrusion machine comprises a chassis. awheel and tooling. The tooling consists principally of a shoe and a die.The chassis supports the wheel for rotation by a motor. An endlessgroove is formed in the periphery of the wheel into which is entrained afeedstock which is commonly a bar of a non-ferrous metal such asaluminium or copper but may comprise metal particles or molten metal.Part of the periphery of the wheel is closely enveloped by the shoe sothat the groove cooperates with the shoe to form a passage in usefeedstock entrained in the groove enters the passage at an open end asthe wheel rotates. The other end of the passage is obstructed by anabutment which is mounted on the shoe and intrudes into the passage.Because the feedstock is confined in the passage and the wheel continuesto rotate, the feedstock is heated by friction with the groove. A die ismounted in a chamber formed in the shoe immediately upstream of theabutment. Eventually the thermal and other stresses imposed on thefeedstock cause the feedstock to extrude through the die.

[0003] The continuous extrusion machine is capable of continuouslyextruding a wide range of sections of non-ferrous metal, for so long asfeedstock is delivered to the groove.

[0004] In order to operate successfully it is necessary to have a smallgap between the periphery of the wheel and the shoe. This gap permits asmall quantity of the feedstock. known as the flash, to extrude out ofthe passage onto the periphery of the wheel and into the gap. The sizeof the gap has a significant effect on the performance of the machine interms of the speed, quality and type of extrusion which can be produced.Conventionally the gap is set before starting the machine. However, whenthe machine is in operation heat causes thermal expansion of the machinecomponents and pressure on the wheel and chassis causes elasticdeformation so that the gap size changes. Thermal expansion typicallyalters the gap by up to 0.7 mm while elastic deformation alters the gapby between 0.3 and 0.5 mm. The effects of thermal expansion andextrusion pressures are non-uniform, will vary during start up, and mayvary during operation and conventionally cannot be measured accurately.

[0005] The elastic deformation is relieved when feedstock ceases toenter the machine, as at shut down, and it is essential that the shoedoes not impinge on the wheel or serious damage will occur. It isconsequently not possible to pre-set the machine to run with a gap ofless than the elastic deformation. It is also disadvantageous that thegap cannot be varied and accurately measured during machine operation inorder to test the performance of various clearances in the production ofan extrusion.

[0006] Accordingly the present invention provides a continuous extrusionmachine having a chassis supporting a wheel for rotation and a shoeenveloping a span of the periphery of the wheel and co-operating with agroove formed in the periphery of the wheel to form a passage, a supportmechanism supporting said shoe and/or wheel to be relativelydisplaceable in a direction perpendicular to the axis of rotation of thewheel during use, a gap sensor system able to sense the size of a gapbetween the wheel periphery and the shoe when the machine is operating,and control means responsive to the gap sensor to adjust the supportmechanism to displace the shoe relative to the wheel.

[0007] The gap sensor system may also sense the shape of the gap.

[0008] In practice it is preferable to support the shoe via the supportmechanism. However, the fundamental objective is to be able toaccurately control the gap size and shape and so the displacement of thewheel relative to the chassis is deemed within the broad concept of thisinvention. Also within the scope of this invention is the displacementof the shoe and the wheel relative to the chassis particularly where itmay be convenient to displace the shoe on one axis and the wheel onanother.

[0009] A preferred support mechanism comprises a hydraulic wedgeassembly having a wedge longitudinally displaceable against acomplementary ramp. The ramp engages and supports the shoe and isconstrained to move in a direction towards or away from the wheel. Bymounting such a support mechanism at a tangent to the wheel so that shoedisplacement is radial it is possible to control the gap size. However,a unidirectional active shoe positioning system is less than whollysatisfactory at least in part because of difficulties in adaptingdifferent shoe types used for radial and tangential mode extrusion andbecause it is desired to control the shape of the gap in addition to itssize. To completely control both the size and the shape of the gap, asindependent variables, it is preferred to provide the support mechanismwith a first and a second wedge assembly. The first wedge assembly isdisposed to displace the shoe in a first direction perpendicular to theaxis of rotation of the wheel and the second wedge assembly is disposedto displace the shoe in a direction perpendicular to the rotary axis ofthe wheel and the first wedge assembly. The directions will ordinarilybe the vertical and horizontal.

[0010] It is preferred that each wedge assembly includes an hydraulicram to longitudinally displace the wedge.

[0011] Although wedges, ramps and rams are thought to be the best way ofimplementing the support mechanism at this time it is conceived that theuse of hydraulic rams alone or ball screw driven rams may be capable ofproviding a support mechanism.

[0012] Means such as Poly-Tetra-Fluoro-Ethylene (PTFE) surfaces may beprovided to reduce the friction between the wedge and the wedge bearing.

[0013] Preferably, where two wedge assemblies are provided to implementa bi-directional dynamic or active shoe positioning process, it ispreferred to provide a gap sensor system having three gap sensors eachlocated peripherally spaced from the other, to sense the size and shapeof the gap.

[0014] An alternative arrangement would be for the shoe to be supportedin the chassis by means of a pivot and swung into position to set thegap size. By supporting the pivot to be displaceable radially via theoperation of a first actuator such as an hydraulic ram, and arrangingfor a second actuator such as a second hydraulic ram to be capable ofswinging the shoe around the pivot, the size and shape of the gap may bedynamically adjusted during machine operation in accordance with thesize and shape of the gap sensed by the gap sensor.

[0015] In order to sense both the size and shape of the gap the gapsensor system will preferably comprise a plurality of gap sensorsdeployed to detect the gap size at positions spaced circumferentiallyaround the wheel.

[0016] Preferably the gap sensor system comprises gap sensors whichsense the gap size directly to avoid the corrections required if the gapsize and shape is sensed indirectly. To this end each gap sensor musttolerate the hostile environment at the interface between the wheel andthe shoe while continuing to measure with accuracy of the order of 0.1mm, so that a gap size of 0.2 mm can be accurately set. The sensor rangewill preferably exceed 0.5 mm to facilitate starting the machine andideally will exceed 1 mm. The preferred form of sensor is a sonic gapsensor.

[0017] A sonic gap sensor relies on the principle that fluid flowthrough an orifice will choke when a fluid pressure upstream of theorifice reaches a critical pressure at which the flow through theorifice is sonic. In this condition the fluid condition downstream ofthe orifice has no influence on the conditions upstream of the orifice.When the orifice is choked the fluid condition upstream of the orificecorrelates with the size of the orifice. By making the gap the orificethe size of the gap can be measured. Thus the gap sensor of the presentinvention consists of at least one port located in the shoe adjacent thegap and a gas delivery pipe for delivering compressed gas to the port ator above the critical pressure. Pressure sensitive transducers aredeployed in the gas delivery pipe in order to sense the gas pressure inthe pipe. Once calibrated, changes in the gas pressures sensed can beused to determine the size of the gap adjacent the port. Thus the gapsize can be determined by coupling the pressure transducers to acomputer or other dedicated processor of the control means.

[0018] Sensors other than sonic gap sensors as presently availablecannot tolerate the environment in the gap for sufficient time to bepractical in a production machine.

[0019] Improvements in the environmental tolerance of such sensors oreven completely new types of sensor would obviously requirereconsideration of the applicability of the sensor to this invention fordirectly sensing the gap size.

[0020] Indirect sensing of the gap size, (i.e. computation of the gapsize from remote measurements) has been contemplated because this avoidsmany of the difficulties inherent in locating a sensor in the hostileenvironment within the gap. Sensors considered potentially suitable forindirect sensing include eddy current sensors, proximity sensors,optical sensors and hall effect sensors. Systems in which the gap sizeis sensed indirectly are considered to be within the broadest scope ofthis invention. Sensors from the previously mentioned list may be usedto sense the gap by sensing the relative positions of the shoe, thewheel and possibly the chassis. Such a system will require the data fromthe indirect sensor(s) to be corrected for thermal and mechanical strainon the, wheel shoe and chassis. While not impossible the difficulties ofcorrection are believed to be more disadvantageous than the difficultiesof directly sensing the gap size.

[0021] The majority of material in the gap is confined to the areas ofthe wheel adjacent to the groove. When using gap sensors wheels 50 mmwider than is conventional are used and the sensors operate at the outer25 mm which is clear of the flash. It is preferred to locate one gapsensor adjacent the mouth of the start of the tooling, one at the centreof the tooling and one immediately downstream of the abutment. So thatthe gap is the only significant constriction in the gap sensor, eachport has a diameter approximately four times the maximum size of thegap. Preferably each gap sensor will comprise one port, overlying theedge of the wheel and communicating with an elongate gas delivery pipe.The gas port pressure (P) may be measured slightly upstream of the ports(e.g. at about 0.05 m) and a delivery pressure (P.) far upstream of (heports (e.g. at about 0.750 m). The ratio of the port pressure to thedelivery pressure is approximately proportional to the size of the gap.

[0022] Conventionally a scraper is required to remove excess flash fromthe wheel rim during machine operation in order to prevent the flashbuild up from fouling the gap as it re-enters the shoe. However,problems arise in setting the scraper position relative to the wheelbecause of thermal expansion and blade tip wear during machine operationwhich alters the relative position of the scraper blade and the wheel.To alleviate this there may be provided a scraper carrier supported forradial displacement toward and away from the wheel rim and supporting ascraper blade at its tip adjacent the wheel rim. The scraper carrier isrendered radially displaceable during machine operation by a device suchas an eccentric shaft, and a motor arranged to rotate the shaft to adegree determined by the control device. The control device responds toa gap sensor mounted on the tip of the scraper carrier to determine theseparation the scraper blade tip and the wheel rim.

[0023] According to another aspect of the present invention there isprovided a method of operating a continuous extrusion machine whereinfeedstock is entrained in a groove formed in the periphery of a wheelrotating in a chassis and drawn into a passage formed between the grooveand a shoe, said passage being obstructed by an abutment supported bythe shoe so that friction between. the shoe and the abutment will causethe feedstock to extrude through a die supported in the shoe, comprisingthe steps of: sensing the actual size of a gap between the wheel and theshoe,

[0024] comparing the actual size of the gap with a predetermined orprevious gap size in a control means to determine if there is adifference, said control means responding to a difference to control asupport structure which supports the shoe and/or the wheel in thechassis to displace the shoe and/or the wheel on at least one axisperpendicular to the axis of rotation of the wheel so that the gap ischanged to reduce the difference.

[0025] The sensing of the gap size and the adjustment of the shoeposition take place during operation of the machine. This may includethe start up operation of the machine before extrusion has begun. As themachine warms up from a cold start, the gap size may be sensedcontinuously but is preferably sensed at intervals. When the gap sizediffers from a previous value, or possibly when it diverges from apredetermined value, a control device of the control means responds toadjust the support structure so that the shoe is moved relative to thewheel to bring the gap size back towards the desired size.

[0026] The desired gap size may be altered during machine operation.Thus while the method contemplates setting the gap size to that requiredfor extrusion, and preventing significant deviation during extrusion, italso contemplates setting the gap size to one predetermined value duringmachine start up, altering that predetermined value during continuousextrusion and possibly further altering the value during shut down ofthe machine.

[0027] The method of sensing the gap size preferably comprises blowingair or another pressurised gas such as an inert gas through the gap atat least one, and possibly two or preferably three circumferentiallyspaced points adjacent the passage. The pressure with which the air isblown is sufficient to ensure that the gap is choked and the pressure ina delivery pipe upstream of the gap can then be sensed and correlatedwith the gap size. It is preferred to sense the pressure and hence thegap at intervals in order to minimise the gas requirement.

[0028] The method may also comprise the steps of sensing the shape ofthe gap, in particular by sensing the size of the gap at two or moreperipherally spaced locations and the step of adjusting the shape of thegap to a desired shape.

[0029] Continuous extrusion machines and a method of operating them,embodying biaxial shoe positioning in accordance with the presentinvention will now be described, by way of example only, with referenceto the accompanying drawings in which,

[0030]FIG. 1 diagrammatically illustrates a continuous extrusion machineset up for radial shoe operation,

[0031]FIG. 2 diagrammatically illustrates a continuous extrusion machineset up for tangential shoe operation,

[0032]FIG. 3 is an enlarged sectional elevation of a portion of thewheel and shoe of the machine where feedstock enters the passage andshowing one gap sensor,

[0033]FIG. 4 is a partly diagrammatic sectional elevation on the lineIV-IV in FIG. 3, on a reduced scale.

[0034]FIG. 5 is a graph showing the calibration of a gap sensor,

[0035]FIG. 6 is a part sectional elevation of the scraper blade assemblyin a machine,

[0036]FIG. 7 is a part sectioned plan view of the scraper blade assemblyof FIG. 5

[0037] With reference to the drawings a continuous extrusion machinecomprises a chassis 1, a wheel 2 mounted in the chassis for rotationabout a horizontal axis, a shoe 3, 3′ a shoe support mechanism,described in detail below and a gap sensor system comprising three sonicgap sensors 4, 4A, 5. The machine is illustrated in the process ofextruding a bar 6 of cast non-ferrous metal. feedstock such as aluminiumor copper. The feedstock is entrained by means of a coining roll 8 in anendless groove 7 formed in the periphery of the wheel 2. As the wheelrotates in the direction of the arrow “A” the bar 6 passes into anenclosed passage formed between the shoe 3, 3′ and the periphery of thewheel 2.

[0038] Movement of the bar 6 through the passage is stopped by anabutment 8. The wheel 2 is rotated by a motor (not shown) so thatfriction heats and compresses the bar 6 until it becomes sufficientlyplastic to extrude out of the passage 7 into tooling 9 which includes adie: In the case of the radial mode of operation shown in FIG. 1 theshoe presents the die so that the extrusion 10 passes from the machineradially with reference to the wheel 2. In the case of the tangentialmode machine shown in FIG. 2 the shoe 3′ is adapted to accommodatetooling 9 which has the extrusion 10′ passing from the machine at atangent to the wheel 2.

[0039] The radial mode machine is best suited to the production ofprofiled sections and tube while the tangential mode is suited tosheathing and cladding a core 11.

[0040] A gap 12 is formed between the periphery of the wheel 2 and theshoe 3 which can be seen enlarged (approximately 10 times larger thanlife) in FIG. 3. The size of the gap 12 during machine operation isoptimally approximately 0.2 mm. During the machine operation some of thematerial of the bar 6 ‘extrudes’ through the gap onto thecircumferential surface of the wheel 2. This material is separated fromthe wheel 2 by means of a scraper assembly 41 shown in detail in FIGS. 6and 7 as described later.

[0041] The wheel 2 and the shoe 3 are subject to deformation cause bymechanical and thermal strain. This deformation tends to increase thegap size during extrusion. The removal of the strain when the feedstocksupply is stopped results in a sudden reduction in the gap size. Themachine must continue to run for a period after the feedstock supply isstopped in order to discharge feedstock from the passage. If the gapsize were of the order of 0.2 mm the sudden reduction in strain causedby the discharge of the passage would cause the wheel to collide withthe shoe resulting in serious damage.

[0042] To alleviate the aforementioned problem the shoe 3 is mounted ona support structure comprising a pair of wedge assemblies, inparticular, a first vertical displacement wedge assembly 13 fordisplacing the shoe 3 vertically and a second horizontal displacementwedge assembly 14 for displacing the shoe 3 horizontally.

[0043] The vertical displacement wedge assembly 13 comprises a basebearing member 15, a wedge 16 disposed with an elongate horizontal facebearing against the bearing member 15 so that an elongate inclined facefaces upwards.

[0044] A ramp 17 has a face inclined at the same angle as the wedge andbearing against the inclined face of the wedge 16. The ramp 17 has ahorizontal face opposite the inclined face which bears against the shoe3. A shim may be interposed between the shoe and the ramp 17. The rampis mounted in the chassis to be displaceable in the vertical directiononly. The wedge 16 and ramp 17 are separated by a low friction spacer(not shown) which may be made of PTFE. Included in the wedge assembly 13is a double acting vertical displacement hydraulic ram 19 connected tothe wedge 16 by a con-rod 20. Hydraulic fluid supply to the extensionchamber of the hydraulic ram 19 is controlled by a right displacementair hydraulic intensifier 21. Hydraulic fluid supply to the retractionchamber of the ram 19 is controlled by a left air hydraulic intensifier22.

[0045] The horizontal wedge assembly 14 comprises a back bearing member23 which is removably secured by pins 23′ into the chassis 1. An innervertical face of the back bearing member 23 provides a bearing surfaceto support a vertical face of a wedge member 24 of the horizontaldisplacement wedge assembly 14. An inclined face of the wedge 24 bearsagainst a complimentarily inclined face of a ramp member 25. The rampmember 25 bears against a vertical face of the shoe 3 and is mounted tobe displaceable horizontally only. A shim may be interposed between theramp 25 and the shoe. A double acting hydraulic ram 26 is linked to thewedge 24 by a con-rod 27. An up air hydraulic intensifier 28 controlsthe delivery of hydraulic fluid to the up hydraulic ram 26. Displacementtransducers 29 monitor the positions of the wedge members 16 and 24 toenable fast movement during start up and shut down.

[0046] Because the wedge 24 must be readily removable from the machinein order to gain access to the shoe 3 it cannot be very rigidly fixed tothe con rod 27. To ensure no backlash in the horizontal movement a downhydraulic ram 30 is provided to impose a constant downward pressure onthe top of the wedge 24. This also helps to ensure smooth movement ofthe wedge by overcoming any stiction which may occur between the wedgeand bearing surfaces despite of friction reducing measures which may beimplemented such as PTFE coatings.

[0047] The air/hydraulic intensifiers deliver a precise volume ofhydraulic fluid every time they are actuated by a pneumatic air signaldelivered to the intensifier.

[0048] Typically the volume may be 2 ml. One stroke from the intensifierwill therefore result in a the wedge attached to the associatedhydraulic ram moving by a single increment resulting in an incrementalshoe movement of typically 0.04 mm. Thus when the control devicecompares a desired gap size with an actual sensed gap size the hydraulicrams can be driven the required number of strokes to achieve the desiredgap size.

[0049] In the radial mode of extrusion shown in FIG. 1 the radial shoe 3forms a passage mostly in an upper quarter segment of the wheel 2. Thepressure imposed on the radial shoe 3 by the feedstock in the passagehas an upwardly directed resultant force. It is therefore necessary toprovide a second down hydraulic ram 31 to urge the shoe 3 down onto thevertical movement wedge assembly 13. An air/hydraulic intensifier 32 isarranged to control the delivery and discharge of hydraulic fluid to thesecond down hydraulic ram 31.

[0050] In the tangential operation mode of FIG. 2 the tangential shoe 31forms the passage in a lower quadrant of the wheel 2. In consequence thepressure applied by the feedstock entrained in the passage includes alarge net downward component on the tangential shoe 31. Although thismakes the second down hydraulic cylinder 31 unnecessary in thetangential mode of operation, the fact that the load on the shoe is nearvertical and has only a small horizontal component makes the provisionof a horizontal shoe displacement ram 31A in the chassis desirable. Thehorizontal shoe displacement ram 31A is mounted in the chassis I andacts directly against the shoe 31 to overcome friction between the shoeand a horizontal support plate 31B by pushing the tangential shoe 31against the ramp 25.

[0051] It will be appreciated from FIGS. 1 and 2 that a singlecontinuous extrusion machine may be adapted by installation of theappropriate radial shoe 3 or tangential shoe 31 to run in either theradial or tangential modes.

[0052] The delivery of air to each air/hydraulic intensifier iscoordinated by a control device (not shown) of the control means, suchas a programmable computer or dedicated processor which cause thedischarge of pneumatic control air from an air reservoir 33 to theair/hydraulic intensifiers via solenoid valves 33A. Rams 31 or 31A arecontinuously pressurised to push the shoe 3 or 3′ against either avertical shoe support plate 31C, or the horizontal shoe support plate31B. The shoe support plates 31B, 31C are each supported by thehorizontal and vertical wedge assemblies 13 and 14. When the wedgeassemblies move the system towards the opposing ram, e.g. the horizontalwedge assembly 13 moves the shoe 3 towards the ram 31 fluid is forcedfrom the ram cylinder through the pressure relief valve and when theshoe is moved away fluid is pumped into the ram 31. Thus a pre-set fluidpressure is maintained in the ram 31 or 31A and corresponding force isapplied to the shoe 3, 3′ to urge it against the wedge assembly 31, 14opposite the ram.

[0053] To summarise cylinders 19 and 26 are master cylinders whichcontrol the position of the wedges and the shoe. Cylinders 30, 31 and31A are slave cylinders which are continuously pressurised to maintain aconstant thrust. If the master cylinders are moved oil is forced in orout of the slave cylinders to maintain the required thrust.

[0054] Each air/hydraulic intensifier is equipped with a microswitchwhich senses each stroke of hydraulic fluid discharge and transmits thisinformation to the control device which can thus deduce the consequentdisplacement of the shoe 3, 3′. The control means in this instance maybe understood to consist of the control device and the pneumatic controlsystem comprising the reservoir 33, the pneumatic valves and theair/hydraulic intensifiers

[0055] The control means is responsive to the size of the gap 12 sensedby the first, second and third gap sensors 4,4A and S. The first gapsensor 4 is located adjacent the entrance to the passage, the second gapsensor 4A is located adjacent the shoe and upstream of the tooling 9 andthe third gap sensor is located downstream of the abutment 8. Each ofthe gap sensors 4,4A and 5 are similar in operation and differsignificantly only in location so only the gap sensor 4 showndiagrammatically in FIGS. 3 and 4 will be described in detail. The gapsensor 4 comprises a gas supply pipe 34 preferably between 0.75 m and2.910 m long. The pipe communicates with a port 35 formed alongside thetooling. The port 35 overlies the rim of the wheel 2 adjacent the groove7. The end of the pipe 34 remote from the gap 12 communicates with asolenoid valve 38. The pipe 34 is of similar diameter to the port 35.The port 35 has a diameter about four times that of the gap size.Pressurised gas is delivered to the solenoid valves 38 from anaccumulator 39 via a pipe 40 and a pressure transducer 37. the pressuretransducer 36 is located near (about 0.05 m) from the port 35. Theoryindicates that measurement of a maximum gap size of 1.375 mm requires aport diameter of 5.5 mm. However, the experimentally derived resultsshown in FIG. 5 indicate that the correlation between the pressure ratioP/Po and gap size is sufficiently linear over a range from 0.2 to 2 mmfor a 5.5 mm.

[0056] To sense the gap size a gas which may be air but may also be anon-oxidising gas such as nitrogen, or a noble gas, is discharged downthe tube 34 at a pressure sufficient to achieve sonic velocity at theaperture 35. As can be seen from FIG. 4, when the aperture is choked andthe flow upstream is subsonic, the ratio of the downstream pressure tothe upstream pressure is dependent mainly upon the size of the gap 12.Since the pressure transducers may be accurate up to +−3447 N/M2 (0.5psi) the gap size may be sensed to an accuracy of about +0.05 mm.

[0057] The pressure transducers 36 and 37 communicate the sensedpressures to the control device where the sensed pressures may beconverted to dimensions and with a pre-set desired gap size. When thecontrol device senses a deviation from the pre-set gap size it issuescontrol signals to the air/hydraulic intensifiers to deliver ordischarge hydraulic fluid from the rams so that the shoe is displaced tobring the gap size back towards the desired size. 4 As can be seen fromexperimentally derived calibration curve of FIG. 4, the pressure ratioP/P. is approximately linear when the inlet pressure P. is 344750 N/M2(50 psi) over a range of gap size from 0-2 mm and the tube length is0.750 mm.

[0058] The calibration of the gap sensor shown in FIG. 4 consists of thefollowing steps, with the wheel stationary and no feedstock in themachine.

[0059] 1. Pre-set the gap at 0.0 mm, this may be determined when P=P0

[0060] 2. Increment the gap by 0.1 mm by applying an appropriate numberof air pulses to the air/hydraulic intensifiers,

[0061] 3. If transducer 37 senses that the pressure in the accumulatoris 344.75 kN/M2 open solenoid valve 38 for 3 seconds.

[0062] 4. Two seconds after opening the valve 38 read the pressures fromtransducer 37 and 38 to the control device.

[0063] 5. Calculate P/P. and PI/P. and map against gap size. 6.Increment the gap by 0.1 mm.

[0064] 7. Repeat steps 1-6 until gap=2 mm.

[0065] Once the gap sensors have been calibrated the operation whenextruding material consists of the steps of.

[0066] 1. With solenoid valves 38 shut, read P. from transducer 38.

[0067] 2. if P0==344.75 N/m2 open valve 38 for three seconds. 3. Twoseconds after valve opens read P and P.′

[0068] 4. Calculate P/P0 and read gap from the calibration map.

[0069] 5. During start up measure the gap every ten seconds.

[0070] 6. During steady running measure the gap every minute.

[0071] 7. If the actual gap size differs significantly from the previousdesired or previous gap size actuate air/hydraulic intensifiers withsufficient pulses to converge the actual gap size to the desired gapsize.

EXAMPLE

[0072] An example of a continuous extrusion machine start procedureusing the previously described continuous extrusion machine requires themachine to extrude through a high pressure die. To achieve this thewedge assemblies 13 and 14 are adjusted so that, when cold, the gap 12has an upstream width of 0.4 mm at an upstream position adjacent thesecond gap sensor 4, an intermediate width of 0.2 mm at an intermediateposition adjacent the second gap sensor 4A and a downstream width of 0.5mm at a downstream position adjacent the third gap sensor 5. The scraperis set to prevent any build up of flash. As the machine starts up themachine temperature approaches 550C and the gap is adjusted until it isparallel with the upstream and downstream gaps set to 0.2 mm.

[0073] The embodiments may be operated automatically by the controldevice responding to signals indicative of the gap size from the firstsecond and third gap sensors. However, the machine may be operatedmanually by an operator observing the appearance and amount of the flashlayer and moving the shoe accordingly.

[0074] Referring no to FIGS. 6 and 7, the scraper assembly 41 comprisesa horizontal support bearing 42 extending parallel to the axis of thewheel 2 to support a scraper carrier 43 which extends substantiallyradially towards the wheel 2. An eccentric shaft 44 extends parallel tothe wheel axis through a bearing block 45 received into a recess in thescraper carrier 43. The eccentric shaft 44 is driven to rotate by ageared motor 46 which by virtue of the eccentric rotation of the shaft44 causes the scraper carrier 43 to be displaced radially toward or awayfrom the wheel 2. A scraper blade 47 is mounted via bolts or any othersuitable device onto the end of the scraper carrier 43 so that when thescraper blade 47 is displaced to a desired position determined by thecontrol the scraper blade 47 removes unwanted flash from the wheel rim.Positioning the scraper blade accurately is important in order toprevent fouling as the wheel rim re-enters the shoe. However, problemsarise in setting the scraper position relative to the wheel because ofthermal expansion and blade tip wear during machine operation whichalters the relative position of the scraper blade 47 and the wheel 2. Toalleviate this problem a sonic gap sensor 48 is mounted on the tip ofthe scraper carrier 43 adjacent the wheel 2. The gap sensor 48 sensesthe separation of the scraper carrier tip and the wheel rim which iscommunicated to the control means which can thus simply determine theactual position of the scraper blade tip relative to the wheel rim.Where there is any difference in the desired and actual position of thescraper blade tip the control steps the motor 46 to reposition thescraper blade tip to reduce the difference. It will be appreciated thatnumerous alternative devices may be employed to achieve displacement ofthe scraper carrier including hydraulic rams, ball screws, worm drivesand rack and pinion drives.

1. A continuous extrusion machine having a chassis (1) supporting awheel (2) for rotation and a shoe (3) enveloping a span of the peripheryof the wheel (2) and cooperating with a groove (7) formed in theperiphery of the wheel (2) to form a passage, a support mechanismsupporting said shoe (3) and/or wheel (2) to be relatively displaceablein a direction perpendicular to the axis of rotation of the wheel (2)during use, a gap sensor system able to sense the size of a gap (12)between the wheel periphery and the shoe (3) when the machine isoperating, and control means responsive to the gap sensor system toadjust the support mechanism to displace the shoe (3) relative to thewheel (2).
 2. A continuous extrusion machine according to claim 1wherein the gap sensor system is able to detect the shape of the gap(12).
 3. A continuous extrusion machine according to claim 1 or claim 2wherein the support mechanism comprises a wedge assembly (13,14) havinga wedge (16,24) longitudinally displaceable against a complementary ramp(17,25).
 4. A continuous extrusion machine according to claim 3 whereinthe support mechanism has a first wedge assembly (13) and a second wedgeassembly (14), the first wedge assembly disposed to displace the shoe(3) in a first direction perpendicular to the axis of rotation of thewheel (2) and the second wedge assembly disposed to displace the shoe(3) in a direction perpendicular to the rotary axis of the wheel (2) andthe first wedge assembly whereby the size and shape of the gap (12) canbe altered during operation.
 5. A continuos extrusion machine accordingto claim 3 or claim 4 wherein each wedge (16,24) is displaced byhydraulic rams (19, 26)
 6. A continuous extrusion machine according toclaim 5 wherein the gap sensor system comprises a gap sensor (4,4A,5)which senses the gap size directly.
 7. A continuous extrusion machineaccording to claim 6 wherein the gap sensor system provides at least twogap sensors (4,4A,5) each located peripherally spaced from the other, tosense the size and shape of the gap (12).
 8. A continuous extrusionmachine according to claim 7 wherein the gap sensor system includes afirst gap sensor (4) located at the entrance to the passage, a secondgap sensor (4A) located immediately upstream of tooling (9) in the shoe(3) and a third gap sensor (5) is located downstream of an abutment (8).9. A continuous extrusion machine according to any one of claims 6 to 8wherein the sensor is a sonic gap sensor.
 10. A continuous extrusionmachine according to any one of the preceding claims wherein a scraperblade (47) is supported on a scraper carrier (43) for radialdisplacement with respect to the rim of the wheel (2), said scrapercarrier (43) being driven by a motor (46) controlled by the controldevice in accordance with signals received from a gap (12) sensor (46)mounted on the carrier to detect the separation of the tip of thescraper blade (47) and the periphery of the wheel (2).
 11. A continuousextrusion machine according to claim 10 wherein the motor displaces thescrap/r carrier (43) by rotation of an eccentric shaft (44).
 12. Acontinuous extrusion machine according to any one of claims 9 to 11wherein the gap sensor (48) is a sonic gap sensor.
 13. A method ofoperating a continuous extrusion machine wherein feedstock is entrainedin a groove (7) formed in the periphery of a wheel (2) rotating in achassis (1) and drawn into a passage formed between the groove (7) and ashoe (3), said passage being obstructed by an abutment supported by theshoe (3) so that friction between the shoe (3) and the abutment willcause the feedstock to extrude through a die supported in the shoe (3),comprising the steps of: sensing the actual size of a gap (12) betweenthe wheel (2) and the shoe (3), comparing the actual size of the gap(12) with a predetermined or previous gap size in a control means todetermine if there is a difference, said control means responding to adifference to control a support structure which supports the shoe (3)and/or the wheel (2) in the chassis (1) to displace the shoe (3) and/orthe wheel (2) on at least one axis perpendicular to the axis of rotationof the wheel (2) so that the gap (12) is changed to reduce thedifference.
 14. A method according to claim 13 wherein the shape of thegap is sensed
 15. A method according to claim 14 wherein thepredetermined gap size is set to a desired gap size while the machine isextruding.
 16. A method according to any one of claims 13 to claim 15wherein the gap size is sensed at at least one position comprising thesteps of: i. blowing a pressurised gas through the gap (12) at at leastone, point adjacent the passage, ii. adjusting the gas pressure to besufficient to that the gap (12) is choked, iii. sensing the gas pressureupstream of the gap (12), iv. communicating the gas pressure to thecontrol means, V. calculating the actual gap (12) size from the gaspressure.
 17. A method according to claim 16 wherein the gap size issensed at at least two circumferentially spaced points adjacent thepassage to determine the shape of the gap (12).